Precision angular indexing servosystem



Oct. 24, 1961 M. FROMER ETAL PRECISION ANGULAR INDEXING sERvosYsTEM 5Sheets-Sheet 1 Filed Dec. 8, 1959 Magro/V Feo/14E@ PUDQLDH /F KEMKASAMUEL E LEN/vox INVENToRs ,QTTORA/EYS Oct. 24, 1961 Filed Deo. 8, 1959M. FROMER ETAL PRECISION ANGULAR INDEXING SERVOSYSTEM /IO'n 5Sheets-Sheet 2 A Y' l/Ob MORTO/v FROME@ RUDOLPA/ /T KFM/(4 5mn/EL FLENA/0X INVENToRs Oct. 24, 1961 M. FROMER ETAL PRECISION ANGULAR INDEXINGsERvosYsTEM 5 Sheets-Sheet 5 Filed Dec. 8. 1959 ,QTTORA/EYS Oct. 24,1.961 M. FRoMER ETAL t 3,005,939

PRECISION ANGULAR INDEXING SERVOSYSTEM Filed Deo. 8, 1959 5 Sheets-Sheet4 SAMUEL E l. ENNOX INVENTORS TTOR/VEYF Oct. 24, 1961 M. FROMER ETAL3,005,939

PREcIsxoN ANGULAR INDEXING sERvosYsTEM Filed Dec. 8, 1959 5 Sheets-Sheet5 Il ll E BRIDGE EXC/ TA T/O/V f -MORTQA/ FRG/wf@ QUDoLP/J FKEMKA.SAMUEL FENNOX INVENTORS TTOQMEYS United States Patent yC) 3,005,939PRECISION ANGULAR INDEXING SERVOSYSTEM Morton Fromer, Maplewood, RudolphF. Kemka, Hillsdale, and Samuel F. Lennox, Oakland, NJ., assignors toGeneral Precision, Ine., a corporation of Delaware Filed Dec. 8, 1959,Ser. No. 858,254 12 Claims. (Cl. S18-28) This invention relates to aprecision angular index stand, and more particularly, to asemi-automatic precision angular indexstand.

i It is often desired to provide an angular index stand which may beused to position a shaft extremely accurately to a plurality ofpredetermined discrete angular positions.A For instance, such an indexstand may be used for the Calibrating of fsyncros, resolvers or othersuch rotary elements and thetesting thereof. In this case, it isnecessary to measure the ratio of input and output voltages at variousaccurately known angular displacements between the rotor and stator ofthe rotary element under test.

In order that such testing and calibration may be accomplished byrelatively unskilled personnel, it is desirable that angular positioningof the index stand be automatic, only necessitating the pushing of abuttonV by the operator. ,j

Furthermore, to insure that the extremely high accuracy of the indexstand is not degraded with use thereof, it is particularly desirablethat the angular position determining elements of the index stan-dinclude no wearing parts. f Y

The index stand contemplated by the present invention is capable ofpositioning .a shaft in discrete increments, such as over a 360 rangewith an accuracy of 2 seconds of arc. The novel angular. positioningdetermining elements of this index stand include no wearing parts, sothat this very high accuracy is not degraded with use.

`lt is therefore an object of this invention to provide an improvedangular index stand'.

It is a further object of this invention to provide means forpositioning a shaft with an extreme degree of accuracy.

lt is a further object of this invention to provide a semi-automaticindex stand.

' It is a further object of this invention to provide highly preciseangular position deter-mining means Iwhich incorporate no wearing parts.

`One feature of the present invention is the use of photoelectric meansfor controlling rough angular positionmg. i

Another feature of the present invention is the use of an improved Ebridge having an extremely sharp characteristic for controlling iinepositioning.

These and 'other objects, features and advantages of the presentinvention Vwill become more apparent from the following detaileddescription thereofv taken together with theV accompanying drawings, inwhich:

FIGUREV f1 is a block diagram of a preferred embodiment of theinvention,

FIGURE 2 is a fragmentary illustration of the index stand, showing thepertinent portions thereof,

FIGURE 3 is a schematic circuit diagram of the pushbutton selector andcontrol means of FIGURE 1,

FIGURE 4 shows the stator element of the E bridge in detail, v

FIGURE 5 shows one of the rotorpelements of the E bridge in detail,

FIGURE 6 shows the electrical and magnetic circuits of the E bridge, and

FIGURE 7 shows'the characteristic curve of the E bridge.

Referring new to FIGURE 1, the index stand itself ICC comprises arotatable shaft on which are tixedly mounted E bridge rotor wheel 102,positioning gear 1104 and program disc `106. Concentric with shaft 100,but not iixed thereto, is mounting adapter 108.

Mounting adapter 108 includes a table for mounting a Syncro under test,which table is independently rotatable through a predetermined angularrange and may be locked in any position within this range. Thi rotorshaft of the syncro under test is xedly coupled to shaft 100. Since thepresent invention is not directed to the mounting adapter, a detailed.description of the structure of mounting adapter 108 is being omitted.

E bridge rotor wheel 102'has secured thereto 72 rotor elements, such asrotor element l10n, at 5 intervals around the circumference of E bridgerotor wheel 102. Each of the 72 rotor elements comes into cooperativerelationship with permanently iixed E bridge stator element 112 during arevolution of wheel 102.

Shaft 100, along with rotor wheel 102 and program disc `106, are rotatedby positioning gear 1104. Positioning gear 104 is driven byanti-backlash motor -114 and servo motor 116. Anti-backlash motor 114,when operated, applies a small torque to positioning gear 104 always inthe same given direction. Servo motor 116 applies a torque topositioning gear 104 which is proportional to the amplitude of theenergization thereof and in a direction determined by the phase of theenergization thereof. Except for very small amplitudes of energizationthereof, the torque produced by servo motor 116 exceeds the torqueproduced by anti-backlash motor 114.

Program disc 106 is an opaque disc having a rst series of spaced holestherein cooperating with light source 118a and photocell 120a, a secondseries of spaced holes therein cooperating with light source 118b andphotocell 120b, and a single hole therein cooperating with light source118e and photocell 120C.

The index stand is selectively `operated and controlled by pushbuttonposition selector and control means 122, and servo motor 116 isenergized through amplifier 124.

Referring now to FIGURE 2, which shows the pertinent portions of theindex stand in somewhat greater detail, shaft 100 is rotatably mountedin housing members 202 and 204 by means of bearings 206 and 208,respectively. Bearings 206 and 208 should be preferably tapered rollerbearings in order to maintain shaft 100 axially rigid.

Detachably secured to E bridge rotor wheel 102, spaced at 5 intervals,are 72 E bridge rotor elements, such as elements 110a, 1-10b, and l10n.

A rotor element 110, which is shown in detail in FIGURE 5, includesmounting holes 502 and 504 and mounting slot 506. For reasons to bedescribed hereinafter, the exact position of each of the rotor elementsis highly critical. Therefore, in mounting a rotor element, the rotorelement is maintained rotatable about the center of mounting hole 502and an eccentric head screw is inserted in mounting hole v504, so thatthe exact position of the rotor element may be adjusted .to the properposition. Screws through mounting hole 502 and mounting slot .506 arethen tightened to maintain the rotor element in its adjusted position.In this manner, the extreme tolerance machining which would be necessaryif the rotor elements were made integral with wheel 102 is eliminated.

As shown in FIGURE 2, positioning gear 104 meshes with drive gear 210 ofanti-backlash motor 114 and with drive gear 212 of servo motor 116.

j Program disc 106, which is opaque, includes a first series of 72'holes, 21451, located at 5 intervals, in cooperative relationship withlight source a and photocell 120a; a second series of 24 lholes 21411,located at 15 intervals, in cooperative relationship with light source11Sb and photo-cell 120i), and a single hole 214e, manifestingreference, in cooperative relationship with light source 110e andphoto-cell 120C. The angular positions of all the holes of program disc106 and the rotor elements 110 are all with respect to this 0 reference.The diameter of each of the holes is quite small, approximately 0.030inch, for example.

Referring now to FIGURE 3, there is included in pushbutton positionselector and control means 122, 0 pushbutton 302, pushbutton 304, and 15pushbutton 306. Associated with 0 pushbutton 302 is start relay 308having normally open contacts 308er, 303C, 308d, and 308f, and normallyclosed contacts 30011, 308e, and 308g. Associated with 5 pushbutton 304is start relay 310 having normally open contacts 3:10a, 310C, 310d, and310i, and normally closed contacts 31011, 310e, and 310g. Associatedwith 15 pushbutton 306 is start relay 312 having normally open contacts312e, 312e, 312d, and 3121, and normally closed contacts 312b, 312e, and312g.

Also included is slow-operate relay 314 having normally open contacts314:1, disconnect relay 316 having normally closed contacts 316s,reversing switch 318 and Thyratron tube circuit 320.

Control means 122 also includes means for applying 60 cycle A.C. powerto the light source, 400 cycle A.C. power to the E bridge andanti-backlash motor as Well as reversing switch 318, and D.C. power tothe relays and Thyratron tube circuit 320. A switch, not shown, may beincluded for turning these power sources off or on.

Glow tube circuit 320 includes Thyratron tube 322, input resistance 324,input capacitance 326, cathode resistance 328 and bias resistances 330and 332.

Low pass filter 334 is inserted in the input leads from the E bridgeoutput.

Referring now to FIGURE 4, which shows a bottom view of the E bridgestator, it will be seen that the central leg 402 of the E bridge stator112 is square in crosssection, and that the end legs 404 and 406 thereofhave a length equal to that of central leg 402 and a width equal toone-half of central leg 402. Furthermore, as shown end legs 404 and 406are offset on opposite sides of central leg 402 with the top edge of endleg 404 in line with the top edge of central leg 402 and the bottom edgeof end leg 406 in line with the bottom edge of central leg 402. v

Surrounding central leg 402 is excitation winding 408 and surroundingend legs 404 and 406, respectively, are output windings 410 and 412,respectively.

Returning again to FIGURE 5, which shows a detail of the top of one Ebridge rotor element 110, there is included central leg 508, having asquare cross-section, and end legs 510 and 512 each having one-half thewidth of central leg 508. However, as opposed to end legs 404 and 406 ofstator element 112, end legs 510 and 512 of a rotor element 110 are notoffset, but are symetrically disposed with respect to central leg 508.

Referring now to FIGURE 6, which shows the electrical and magneticcircuits of E bridge stator element 112 in cooperative relationship withan E bridge rotor element 110, excitation winding 408 surrounds centralleg 402 of statorelement 402, output winding 410 surrounds end leg 404of stator element 402, and output winding 412 surrounds end leg 406 ofstator element 402. As shown, output windings 410 and 412 are wound inopposite directions and are connected in series with each other. Thearrows show the magnetic circuit between central legs 402 and 508 andend legs 404 and 510 and end legs 406 and 512, respectively.

It will be seen that the amplitude of the E bridge output will be thedifference between the amplitudes of the voltage induced in outputwinding 410 and the voltage induced in output winding 412. The amplitudeof the voltage induced in output winding 410 depends upon the reluctanceof the magnetic circuit therethrough and the amplitude of the outputvoltage induced in output winding 412 also depends upon the reluctanceof the magnetic circuit therethrough. Both the absolute and relativevalues of these respective reluctances depends upon the exact angularpositioning between a rotor element and stator element 112. Because ofthe relative sizes, shapes and orientation of legs 402, 404 and 406 ofstator element 112, described above, the characteristic curve, shown inFIGURE 7, of the output voltage from the E bridge vs. angular deviationbetween a rotor element 110 and stator element 112 is extremely sharp.

Considering now the operation of the device, it will be seen fromFIGURES l and 3 that 60 cycle A.C. excitation is applied to lightsources 118a, 118b and 110C, causing these light sources to emit lightbeams. Also 400 cycle excitation is applied to the winding of thecentral leg of E bridge stator element 112 and is also applied toanti-backlash motor 114. In response thereto, antibacklash motor 114applies a small torque to positioning gear 104 in a given direction,which for the purposes of this discussion will be assumed to be in acounterclockwise direction.

Assume that originally the index stand is in its zero degree referenceposition and that 5 pushbutton 304 is manually momentarily closed. Inresponse to the closure of 5 pushbutton 304, operating ground for startrelay 310 is applied thereto through normally closed contacts 316:1,312b, 31011, and 30gb and the closed contacts of 5 pushbutton 304.

Therefore, start relay 310 operates to effect the closure of normallyopen contacts 310a, 310e, 3100. and 310], and the opening of normallyclosed contacts 3100, 310e and 310g.

The closure of contacts 310e and the opening of contacts 310:5, whichare make-before-break contacts, transfers the operating ground directlyto start relay 310, holding start relay 310 operated and rendering allof the pushbuttons ineffective so long as relay 310 remains operated.Thus, only one of start relays 308, 310, and 312 may be operated at atime.

The closure of contacts 310C applies operating ground to slow operaterelay 314. However, relay 314 does not operate immediately due to itsslow operate characteristics.

The closure of contacts 310d applies a resistance ground to 5 photocell120e from ground through resistance 330, normally closed contacts 308e,operated contacts 310d and the 5 conductor to the photocell -120a.However, the circuit path for photocell remains incomplete because thecommon lead from the photocells remains open at open contacts 314:1.

The opening of contacts 310e removes a bias potential from the controlelectrode of Thyratron tube 322, which will be described in greaterdetail hereinafter.

The opening of contacts 310g and the closure of contacts 310f transfersthe input to servomotor 116 through amplifier 124 from the output of theE bridge to the 400 cycle source through reversing switch 318. The phaseof the 400 cycle energization, and hence the direction of the torque-applied by servo motor 116 to shaft 100 through positioning gear 104,is determined by the switch position of reversing switch 318. For thepurposes of the following discussion it vw'll be assumed that reversingswitch 318 is in its left position, and that in this position the torqueapplied by servo motor 116 is in a counter clockwise direction.

Thus, both servo motor 116, which applies a relatively large torque, andanti-backlash motor 114, which applies a relatively small torque, applytorques in a counterclock- Wise direction. Therefore, shaft 100 alongwith E bridge rotor wheel 102 and program disc 106 commences to rotatefrom its zero degree reference position in a counterclockwise direction`at an angular velocity proportional to the sum of the torques fromservo motor 116 and antibacklash motor 114.

While shaft 100 remained in its zero degree reference position, theholes .manifesting zero degrees of the first serie's'of holes,the'second series of holes and hole manifesting z ero degrees of programdisc 196 were respectively, in cooperative relationship with photocells120a, 120b and 120e, respectively. However, at this time photocells120s, 4120b and 120e were unoperated, although a circuit path forphotocell 120' was prepared, but not completed, by the closure ofcontacts 310:1, as previously described.

When program disc 19o has rotated a small distance, so that the holesare no longer in cooperative relationship with the photocells, slowoperate relay 31-4 finally operates, closing normally open contacts 314ethereof, thereby completing the circuit path only for photocell 124m.Now, however, an opaque portion of program disc 106'is between lightsource i18n and photocell 12ta, so that photocell 120e remainsquiescent.

When shaft 101i and program disc 106 have rotated through approximately5, the next hole in the irst series of holes comes into cooperativerelationship with photocell 126, light from light source 118e reachesphotocell 1200, and photocell 12Go conducts. This, in turn, causes thepotential on theco-ntrol electrode of Thyratron tube 322` to -becomemore positive, andfThyratron tube 322 fires, conducting a relativelyheavyl currentv through disconnect relay 3:16 and cathode resistance328.

The conduction currentthrough cathode resistance 328 raises thepotential of the cathode of Thyratron tube 322 to the point where theanode-cathode voltage across Thyratron tube 322 is insufficient withoutthe relatively positive potential on the control electrode thereof tomaintain Thyratron tube 322 conducting.

The conduction current through disconnect relay 316 causes relay 316 tooperate, thereby openingv normally closed contacts 31661 thereof. Theopening of contacts 316:1 breaks the previously -described operatingground holding start relay 310 operated. Therefore, start relay 310restores.V

In'response to the restoration ofrelay 310, contacts 310C reopen,de-energizing relay 314, which restores to open contacts 314@ thereof.

, Furthermore, contacts 310d are reopened, breaking the previouslydescribed circuit path for photocell 12021, and contacts 316e arereclosed, applying ground to the control electrode of Thyratron tube 322through resistance 330, normally closed contacts 308e, 310e, 312e andresistance 332. Breaking the circuit path for photocell 1.20ct removesthe relatively highV positive potentialon the control electrode ofThyratron tube 322, and resistances 330, 332, 324, and 3218. form avoltage divider for placing a relatively low bias potential on thecontrol electrode of Thyratron tube 322. Since, as lpreviouslydescribed, the anodecathode potential is too lo-w, due to the conductiondrop across cathode resistance 328, to maintain Thyratron tube 322conducting Without a relatively high positive potential on the'controlelectrode thereof, Thyratron tube 322 is nowrdeactivated. Therefore,disconnect relay 3,16 restores and contacts 316e thereof reclose. A

The reopening of contacts 3107" and the reclosing of contacts 310gtransfersA the input to servo motor 4116 through amplifier 124 from the40G cycle AC. source to the output from the E bridge, which is appliedthrough low pass ilter 334.

Since servo motor 1.16lhas been assumed to have appliedaAcounter-clockwise torque, at the moment the E bridge output is appliedto servo motor 1-16, A6 in FIGURE 7 will be to the right of the nullpoint of the characteristic of the E bridge.V In thiscase, the phase ofthe E bridge output will be such as to cause servo motor 116 to continueto apply -a counter-clockwise torque. Therefore, shaft 100 willrberotated in a counter-clockwise direction, causing A0 to'move to theAleft in FIGURE 7 toward the null point on the characteristic curve, theoutput Vfrorn the E bridge thereby decreasingtoward zero. At the nullthe single Y point, servo motor 1-16 applies zero torque to shaft 1th).However, anti-backlash motor 114 still applies a small counter-clockwisetorque to shaft 100, so that shaft 1u() continues to rotate in acounter-clockwise direction beyond the null point of the E bridge. Thisresults in reversal in the phase of the output of the E bridge, causingservo motor 116 to now apply a clockwise torque to shaft 100 which risesvery rapidly due to the sharp characteristic curve of FIGURE 7. When theclockwise torque of servo motor 116 is exactly equal and opposite to thecounterclockwise torqueof anti-backlash motor 114, shaft 100 comes torest.

The purpose of inserting low pass filter 334 in the output of the Ebridge is in order to filter out harmonics, particularly the second andother even harmonics. Since the output from the E bridge is obtainedfrom output winding 410 connected in series opposition with outputwinding 412, as shown in FIGURE 6, and even harmonics are 180 out ofphase With the fundamental, the amplitude of the second harmonicappearing in the output of the E bridge reaches a maximum when thefundamental is at null. It is, therefore, necessary to lter out theharmonics in order not to destroy the extreme accuracy of the E bridge.

If now, the operator should press 5Yu pushbutton 364 again, theoperation will be repeated and the index will move an additional 5. Y

If the operator should press either 15 pushbutton 395 or 0 pushbutton302, the operation will be identical in all respects except thatphotocell 12% or 120C, as the case may be, will be rendered operated,rather than photocell 12011. Therefore, the 400 cycle A.C. energizationof servo motor 116 will drive shaft 1M either to the next hole incooperative relationship with 15 photocell 12012 or to the single 0reference hole, as the case may be, before the output of the E bridgetakes control.

If reversing switch 318 is switched toits right position, the 400 cycleA.C. energization of servo motor 116 will have a phase such that servomotor 116 applies a clockwise torque. In this case, shaft 160 has atotal clockwise torque applied thereto which is the difference betweenthe relatively large clockwise torque of servo motor 116 and therelatively small counter-clockwise torque of anti-backlash motor 114. Itwill be seen that in this situation, at the moment the E bridge assumescontrol, A0 of FIGURE 7 is to the left, and then moves toward the right.Shaft 100, therefore, comes to rest at the same realtive point of FIGURE7 as previously described, i.e., Where the torques applied by servomotor 116 and anti-backlash motor 114 are equal and opposite. Thus, thepresence of anti-backlash motor 114 ensures that the accuracy ofpositioning is not affected by a dead space in the region of the null,due to the inherent I:friction in servo motor 116, and that shaft willcome to rest at the same point regardless of whether it approaches thispoint from a clock-wise or counter-clockwise direction; v

As mentioned earlier, the present index stand operates with an-error ofno more than two seconds of arc in positioning shaft 100 to anyparticular one of the 72 discrete positions. This extremely highdegreeof accuracy is a result of the initial positioning of each ofthe-72 rotor elements on E bridge rotor wheel 102 and the relative size,shape and orientation of legs 402, 404 and 406 of stator element 112,and legs S08', 510 and 512 of the rotor elements 110. Considering nowthe manner inwhich each of the 72 rotor elements 110 are initiallypositioned on E bridge rotor Wheel 102, a precise vangle measuringstandard whichV can measure angles accurately to less than a second ofarc, is coupled to shaft 110. Then 0 pushbutton 332 is pressed to rotateshaft 100 to its zero'degree reference position, and the angular readingon the standard is noted. The 5 pushbutton 304 .is then pressed, `andwhen shaft 100 comes to rest, the angular reading on the standard isagain noted. The difference between the two readings should be exactlyIf it is not, the eccentric screw through hole 504 is adjusted toslightly shift the position of the rotor element 110 manifesting 5 in adirection to minimize the error. This entire process is repeated againand again, in cut and try fashion, until the error is eliminated. Thenthis rotor element is permanently held 1n its proper position bytightening the screws through hole 502 and slot 506. In a similarmanner, each of the 70 other rotor elements is adjusted to itsrespective proper position.

It will be seen that the relative size, shape and orientation of legs508, 510, and 512 of a rotor element 110 and legs 402, 404 and `406 ofstator element 112, provides a large change in coupling area betweenrotor and stator elements in response to a small relative angular shiftbetween these elements. Furthermore, it will be seen that the change incoupling area between legs 404 and 510 is in one direction and thechange in coupling area between legs 406 and 512 is in lthe oppositedirection, i.e., when the coupling area between legs 4M and 510'increases, the coupling area between legs 406 and 512 decreases, andvice versa. This results in providing the very sharp characteristiccurve of FIGURE 7. Because this characteristic curve is so sharp, theangular resolution of the disclosed E bridge is very high, and hence theerror in angular positioning is very low, no more than two second ofarc, as stated previously.

Although only a preferred embodiment of the invention has been describedin detail herein, it is not intended that the invention be restrictedthereto, but that it be limited only by the true spirit and scope of theappended claims.

We claim:

1. A precision angular index stand for accurately positioning a shaft toany one of a discrete plurality of angular indices, said index standincluding E bridge means and rough positioning means associated witheach of said angular indices, torque means for applying a torque to saidshaft of a magnitude determined by the amplitude of an input signalapplied to said torque means and in a direction determined by the phaseof said input signal, means for applying a constant amplitude signal ofa given phase as said input signals to said torque means, whereby saidshaft is caused to rotate in a given direction, and control meansresponsive to said rough positioning means for transferring said inputsignal from said constant amplitude signal to the output from said Ebridge means in response to said shaft being rotated to a point in thevicinity of one of said angular indices.

2. The index stand defined in claim 1, wherein said E bridge meanscomprises a wheel mounted to said shaft on which there are disposed atpredetermined angular intervals individual E bridge rotor elements, anda fixed E bridge stator element with which each of said rotor elementscomes into cooperative vrelationship as said shaft rotates.

3. The index stand defined in claim 2, wherein each of said rotorelements and said stator element includes a central leg and two endlegs, said central legs being of rectangular cross-section and having agiven width, sa1d end legs being of rectangular cross-section and havinga width equal to one-half that of said central legs, said end legs ofeach of said rotor elements having a first orientation with respect tosaid central leg thereof and said end legs of said stator element havinga second orientation with respect to said central leg thereof, in one ofsaid first and second orientations the bisector of the width of saidcentral leg is also the bisector of the width of said end legs and inthe other of said first and second orientations the bisector of thewidth of one end leg is offset one-half the width thereof on one side ofthe bisector of the width of said central leg and the bisector of thewidth of the other end Ileg is offset one-half the width thereof on theother side of the bisector of the width of said central leg.

Q o 4. The index stand defined in claim 3, wherein said stator elementincludes an euergization winding wound around each end leg thereof, and-means for connecting said output windings in series opposition forobtaining the output of the E bridge.

5. The index stand defined in claim 4, further including a low passfilter coupled to the output of said E bridge.

6. The index stand defined in claim 1, wherein said rough positioningmeans comprises an opaque program disc mounted to said shaft, a seriesof small holes through said opaque disc spaced at predetermined angularintervals, a fixed light source located on one side of said disc withwhich each of said holes comes into cooperative relationship as saidshaft rotates, and a photocell located on the other side of said discwhich is oriented to come into cooperative relationship with said lightsource each time one of said holes passes therebetween.

7. The index stand defined in claim 6, wherein said control meansincludes means operated in response to light from said light sourcepassing through a hole and impinging on said .photocell for transferringsaid input signal from said constant amplitude signal to the output fromsaid E bridge means.

8. The index stand defined in claim 6, wherein said disc includes aplurality of series of small holes, each of said series of spaced holesbeing spaced at different predetermined angular intervals, andrespective light sources and photocells on opposite sides of said discin cooperative relationship with each respective series of spaced holes,and wherein said control means includes means for selectively renderingoperative any one of said photocells, and means responsive to light fromsaid light source impinging on said photocell which has been renderedoperative through a hole of that series of holes which is in cooperativerelationship with the respective photocell which has been renderedoperative for transferring said input signal from said constantamplitude signal to the output from said E bridge means.

9. The index stand defined in claim 8, wherein said holes of a firstseries `are spaced at regular angular intervals over a 360 range withrespect to a zero degree reference position, wherein the angularinterval between holes of each other series is a different multiple ofthe angular" interval between said first series of holes and is measuredfrom the same zero degree reference position, and wherein said E bridge`means includes a wheel mounted to said shaft on which there aredisposed at angular intervals corresponding to the angular intervalsbetween lsaid first series of holes individual E bridge rotor elements,and a fixed E bridge stator element with which each of said rotorelements comes into cooperative relationship as said shaft rotates.

10. The index stand defined in claim 1, further including second torquemeans for applying a constant second torque in a given direction to saidshaft, said torque being relatively low compared to the torque providedby said first mentioned torque means except when the input signal tosaid yfirst mentioned torque means is the output from said E bridgemeans which is in the immediate vicinity of the null thereof.

211. The index stand defined in claim 10, lfurther including means forreversing the phase of the constant amplitude signal applied as theinput signal to said first mentioned torque means.

.12. The index stand defined in claim 11, wherein said first mentionedytorque means is a servo motor and said second torque means is ananti-backlash motor.

References Cited in the file of this patent UNITED STATES PATENTS2,415,819l Halpert et al Feb. 18, 1947 2,484,022 Esval Oct. 11, 19492,862,193 Fryklund Nov. 25, 1958

